The experience of pain relief is closely associated with the affective-cognitive processing of hedonic value. Higher pain relief plays a key role in patients' satisfaction with treatment and higher motivation to receive treatment. Previous studies focused on pain and anxiety of endodontic treatment. However, the association between pain relief, pain, and anxiety has not been investigated. We investigated 35 patients (20 females and 15 males, with mean age 46.3years old) with symptomatic irreversible pulpitis receiving emergent endodontic treatment. Pain-related experience, including pain relief, pain, and anxiety, was assessed before and after treatment, based on an 11-point numerical scale. We found that before treatment, the expected pain relief (median score = 8) that one anticipated was positively correlated with the degree of pain reduction perceived after treatment (two-tailed rho = 0.51, p = 0.002). Expected pain relief was also positively correlated with the post-treatment relief (two-tailed rho = 0.76, p < 0.001) and the relief recalled after 6weeks (two-tailed rho = 0.69, p < 0.001). Moreover, logistic regression models revealed that expected pain relief was a significant predictor of the subgroups of low vs. moderate scores of post-treatment pain (B = - 1.06, p = 0.017) and anxiety (B = - 1.60, p = 0.009), controlled for the condition of pre-treatment medication. The current findings highlight that for patients with symptomatic irreversible pulpitis who received emergent endodontic treatment, their expected pain relief before treatment is a critical factor associated with pain reduction and post-treatment pain/anxiety. Assessment of expected pain relief may be integrated into pre-treatment assessment for painful dental patients.